A P-I-N photovoltaic device typically comprises a layer of P-type conductivity semiconductor material, a layer of substantially intrinsic (I-type) conductivity semiconductor material and a layer of N-type conductivity semiconductor material with electrical contacts being made to the P and N-type conductivity layers. Carlson, in U.S. Pat. No. 4,064,521, incorporated herein by reference, has disclosed photovoltaic devices, including P-I-N devices, fabricated in a glow discharge containing silane (SiH.sub.4). The deposited material, comprising hydrogenated amorphous silicon (a-Si:H), exhibits an energy bandgap of about 1.7 electron volts (eV), depending upon the deposition conditions. Pankove in U.S. Pat. No. 4,109,271, incorporated herein by reference, has disclosed the use of a wide bandgap hydrogenated amorphous silicon carbon alloy (a-SiC) in the doped layer through which light enters the device. The bandgap of the a-SiC alloy varies between about 1.7 and 3.2 eV depending upon the alloy composition.
The efficiency of these devices for the generation of electrical power depends upon the current collection and the open circuit voltage (V.sub.oc). Current collection has been improved using light-trapping techniques and by providing an I-type layer whose bandgap energy decreases with distance from the light-entry surface. The decreasing bandgap energy provides both an internal electric field which enhances the collection of charge carriers and an increased absorption of the incident light. An increase in V.sub.oc has been observed when an a-SiC:H alloy is used in place of a-Si:H for the P-type layer through which light enters the device. Further increases in the voltage output are possible using a tandem arrangement of individual cells. However, it would still be desirable to improve the performance of the individual cells by changes in their structure or the materials used to fabricate the devices.